This invention relates to lead acid storage batteries and more particularly to lead acid storage batteries wherein the electrolyte is an immobilized electrolyte paste.
The lead acid storage battery, which converts chemical energy into electrical energy, generally consists of positive and negative plates, separators between the plates, a dilute sulfuric acid as the electrolyte, and a battery case wherein the preceding elements are positioned in the case.
The reversible reaction is basically as follows: EQU PbO.sub.2 + Pb + 2H.sub.2 SO.sub.4 .revreaction. 2PbSO.sub.4 + 2H.sub.2 O
During charging the equation proceeds from right to left and during discharging from left to right.
Several drawbacks with these batteries are from the standpoint of safety and maintenance. Care must be exercised in handling these batteries to prevent spillage of the fluid sulfuic acid electrolyte since this electrolyte is a hazardous material. Also the batteries require continuous periodic maintenance of adding makeup water to the electrolyte due to the evaporation of water from the sulfuric acid electrolyte.
Various additives are known which will immobilize the electrolyte and thus decrease the hazards of spillage and decrease the amount of maintenance by inhibiting water evaporation from the sulfuric acid electrolyte. These methods have, however, accompanying disadvantages.
One such additive is very fine, sub-micron, silica particles with a diameter of the order of 0.02 .mu.. These sub-micron silica particles immobilize the electrolyte by forming a gel. A disadvantage with this additive is in the handling of the silica before it is admixed with the electrolyte. Because of the extreme fineness of this material it readily becomes airborne and measures must be taken to confine it. Furthermore, care must be exercised to prevent inhalation of this material due to health hazards.
Other additives which have been used to immobilize sulfuric acid electrolytes are organic polymers containing hydrophilic functional groups such as polyacrylamide, polyvinyl alcohol, and copolymers of maleic anhydride and methyl vinyl ether.
These organic polymers immobilize the electrolyte by forming gels since they contain hydrophilic functional groups capable of partial solubility in the electrolyte. The disadvantage, however, of these partially soluble gel forming polymers is that because of their hydrophilic functionality, they also break down in the electrolyte. Their life as an immobilizer is, therefore short and upon breaking down they also pollute the electrolyte.
Prior to this invention, organic polymers which are stable in the sulfuric acid electrolyte, since they do not contain hydrophilic functional groups, have, previously, not been considered useful for this purpose because they correspondingly do not gel to immobilize the electrolyte.
Sodium silicate, water glass, has also long been known to gel and thereby immobilize sulfuric acid electrolytes. Only a high purity, chlorine free sodium silicate, however, may be used since the chlorine damages the effectiveness of the cell. Furthemore, sodium silicate which is sufficiently chlorine free is expensive and difficult to obtain.